Different rocks have different properties. Measuring these differences provides information about the distribution and the structure of the rocks on the surface and at depth, and gives a 3-dimensional picture of the earth’s crust. Scientists use this information to better understand the evolution and structure of the Earth and to more effectively explore for mineral, hydrocarbon, and water resources.
• What is geophysics?
• Aeromagnetic surveys in central and western Saudi
What is geophysics?
Geophysics is the study of the Earth by quantitative physical methods – principally by measuring the gravitational, magnetic, electrical, and seismic-velocity properties of the Earth’s surface and interior.
Different rocks have different properties. Measuring these differences provides information about the distribution and the structure of the rocks on the surface and at depth, and gives a 3-dimensional picture of the earth’s crust. Scientists use this information to better understand the evolution and structure of the Earth and to more effectively explore for mineral, hydrocarbon, and water resources.
What are the most common types of surveys?
These include:
• Gravity surveys
• Magnetic surveys
• Electrical surveys
• Electromagnetic surveys
• Radiometric surveys
• Seismic surveys
• Radar surveys
• Downhole electrical and caliper surveys
Surveys are done, for different purposes, at regional scales (covering large areas, both on the ground and from aircraft) and at local scales (covering specific sites such as mineral prospects, again on the ground or with airborne instruments, or done from within drillholes deep in the ground).
What has been done in Saudi Arabia?
Geophysical surveys have been done in Saudi Arabia since the 1920s.
• In the east, they include airborne and ground-based magnetic and gravity survey, seismic surveys, and downhole surveys and have been done for oil exploration. The data are held mainly by Saudi Aramco.
• In the central and western parts of the country, airborne magnetic and radiometric and ground-based gravity surveys have been done for regional studies of the structure of the Precambrian Arabian shield and adjacent Phanerozoic sedimentary rocks. Many other surveys have been done for mineral exploration purposes, and the data are held by the Saudi Geological Survey.
• Other types of important geophysical investigations include those done by Universities and other agencies that focus on elucidating the deep crustal structure of the Arabian Plate and Red Sea.
Aeromagnetic data
Airborne regional magnetic surveys cover outcropping Precambrian rocks in the western part of Saudi Arabia (the Arabian Shield), Phanerozoic sedimentary rocks east and north of the shield (the “Cover rocks”), and parts of the Red Sea basin.
Magnetic data (reduced to the pole) for central and western Saudi Arabia--an example
of the type of magnetic data available from SGS
Radiometric data
SGS has radiometric (U, K, Th) covering part of the Phanerozoic rocks adjacent to the Arabian shield, and U,K, Th data for parts of the shield. The data reveal subtle changes in the lithologies of the Phanerozoic rocks and help to clarify the stratigraphic succession.
Composite false-color image of U, K, and Th data for the Ar Riyad area
Gravity data
Gravity data were acquired in the 1980s by the predecessor of SGS for the southern and smaller northeastern parts of the shield and for some areas adjacent to the Red Sea. Because of the wide spacing of data points, the data are only applicable to regional interpretation but, nonetheless, provide an important picture of the structure of the Arabian shield and Red Sea coast and, in conjunction with aeromagnetic data, are useful for metallogenic interpretations.
Aeromagnetic surveys in central and western Saudi Arabia
Applications of geophysics by SGS
Geophysical surveying is one of the important functions of the Saudi Geological Survey and has applications in many aspects of work by the Saudi Geologic Survey. It is used to locate small physical targets such as pipe-lines and concentrations of metallic ores; to locate more subtle targets such as zones of clay alteration around mineral deposits or the boundaries and thicknesses of underground water reservoirs; and to investigate large-scale phenomena such as the potential movement of magma in the Earth’s crust, the drift of the Arabian Peninsula as it moves away from Africa, and the sources and intensities of earthquakes that affect the region.
In exploration geophysics, the main objective is to map structures that are of potential economic importance, such as those that control the location of ore deposits and petroleum reservoirs or to define the character of an aquifer. In geologic mapping, geophysics is commonly used to differentiate rock types and characterize their contacts. In the fields of geohazards and the environment, geophysics is used to measure features such as the magnitude and location of earthquakes and earthquake epicenters, and the levels of naturally occurring radiation. During the past seven decades, geophysical methods contributed significantly to geologic mapping and exploration for hydrocarbons, minerals and aquifers in Saudi Arabia, and aided the discovery of several ore deposits of economic significance in the Arabian shield.
Regional aeromagnetic surveys
Regional and more detailed geophysical surveys have been conducted in central and western Saudi Arabia for more than four decades. They were mainly done under the auspices of the Deputy Ministry for Mineral Resources and the results are now archived for safekeeping and further processing by the Saudi Geological Survey. The most complete geophysical coverage of western Saudi Arabia is provided by aeromagnetic data acquired in surveys between 1962 and 1984. These surveys were regional in nature, and the nominal survey altitudes were up to 300 m above the ground. The survey data have been recently merged to form a regional composite magnetic dataset for the central and western parts of the country and are shown here as maps of the total field reduced to the pole and a gray-scale map of the first vertical derivative of the total field reduced to the pole.
Area covered by aeromagnetic surveys in
central and western Saudi Arabia
Principles of magnetic surveying
Magnetic surveys measure spatial variations in the magnetic field. The results reflect variations in the magnetic properties of the underlying rocks, and provide valuable information about the composition and structure of the crust. The measurements are typically shown as maps of magnetic anomalies, that is areas in which the recorded intensity of the magnetic field is higher or lower than the regional norm. Because present-day surveys acquire data in digital form, the results may be treated by a range of different mathematical processes to create a large range of different types of highly useful maps.
Most rocks contain at least a small amount of magnetic minerals, such as iron oxides or ilmenite. The Earth's magnetic field induces secondary fields in the rocks in the direction of the main field, and the anomalous fields can be measured at the surface and used to make inferences about the nature of the underlying lithology and structure. There may also be permanent remanent magnetization in the rocks, which can have an orientation that differs from the present field direction. At temperatures greater than the Curie temperature, which is about 550-580°C, materials change from ferromagnetic to paramagnetic behavior. This corresponds to depths in the lower crust or upper mantle, below which the rocks should not contribute to the observed magnetic anomaly field. While the strongest magnetic effects are generally associated with igneous lithologies, especially basic rocks such as basalts, even sedimentary rocks may give a low-level magnetic response measurable by modern techniques. The unit of measurement of field strength for magnetic surveying is the nanotesla (nT), previously known as the gamma in the older literature. The magnetic field strength in central Saudi Arabia is about 41000 nT, while in surveying the field is measured to 0.1 nT or better. Minor anomalies of geological interest may be less than 1 nT in amplitude. Cesium vapor or similar magnetometers are used in aircraft, although proton precession magnetometers may also be employed in ground surveys.
Typically one or more magnetometers are used on an aircraft to survey large areas in a systematic pattern of flight lines, and measurements on the ground may be used for detailed studies of very localized areas, such as in mining applications. Two or more magnetometers at a fixed separation on an aircraft can be used to measure the field gradient directly, which may be useful in delineating complex magnetic patterns. Since the amplitude of an anomaly tends to decrease rapidly with distance from the source, the observations should be taken as close to the surface as possible. Some recent aeromagnetic surveys elsewhere have therefore been flown at altitudes as low as 20 m above the ground, with sample intervals of 0.1 second (or even less), or about 6 m. Earlier surveys tended to have significant navigational problems, but with the advent of GPS it is now possible to obtain very accurate flight line patterns, which in conjunction with improved processing techniques has resulted in a great improvement in data quality.
The Earth's idealized International Geomagnetic Reference Field (IGRF) is generally subtracted from the measured field intensity to remove regional trends and give an anomaly or residual field that is largely an expression of the local geologic structure. In addition to image enhancements to reveal details in magnetic anomaly images, forward modeling of magnetic grid or profile data is often employed to determine geological structure. Automated inversion techniques may be used to model magnetic data over large areas, including mapping source depths and outlining structural boundaries.
A new aeromagnetic compilation
The magnetic anomaly map shown here is a new compilation completed in 2002 of aeromagnetic data held by the Saudi Geological Survey for central and western Saudi Arabia.
• Most of the western part of Saudi Arabia, covering the Precambrian rocks of the Arabian shield, was covered by 5 large surveys flown in the 1960s at 800 m line spacing, at an altitude of 150 or 300 m, and totaling about 768,000 line km.
• Smaller areas within the shield, covering Tertiary flood basalt or “harrats”, were flown in the 1980s.
• The central Red Sea was surveyed in 1976-77 as part of the work of the Saudi-Sudanese Red Sea Commission. It is included in its entirety here due to the significance of the data in the analysis of regional tectonics. The sample interval for all the data was 1 second, or about 60 to 70 m along the profiles.
• The Phanerozoic 'Cover” rocks around the shield and along parts of the Red Sea coastal plain were covered by an aerial survey totaling 490,000 line-km in 1982-83, flown mostly at a line spacing of 2 km and a nominal altitude of 120 m above the ground. The survey overlapped a small part of the southeast shield grid, duplicating part of the previously done shield survey.
Individual survey areas used in the new compilation
To make the new compilation, the original profile data for the surveys were leveled, gridded, and merged. The leveling, thoroughly done in 2002 for the first time on central and western Saudi Arabian data, results in a significant reduction in “noise” compared with earlier renditions of the aeromagnetic data and a consequent improvement in the amount of detail discerned. Merging was done on the grids, not the line data, and involved local warping of the grids to minimize offsets at the survey boundaries. Earlier processing of the shield data in the 1960s and 1970s made incorrect IGRF adjustments and resulted in a marked regional gradient that detracts from the usefulness of some of the previously published magnetic maps of western Saudi Arabia. Processing of the Cover Rock data in the 1980s, however, used a more reasonable IGRF that caused no appreciable gradient and yielded a “balanced” appearance for the Cover Rock survey maps. Merging the shield grid with the Cover Rock grid effectively applied the Cover Rock IGRF to the shield data and removed the gradient previously present in the shield maps.
The grid interval is 200 m, which is fine enough to retain subtle detail in the data, and is appropriate for the line spacing of most of the shield surveys. Unlike some earlier published versions of the magnetic data, no upward continuation has been applied since the processing removed virtually all the leveling errors and produced minimal discontinuities at the survey boundaries. The new compilation retains, therefore, more fine detail than previous compilations.
Reduction to the pole (RTP) was carried out using standard Fourier-based procedures, where ideally the RTP grid should show anomalies directly over the causative body, rather than as displaced dipolar anomalies in a total magnetic intensity (TMI) grid. Dipole effects are particularly evident at low magnetic inclinations. For an isolated dipole anomaly, which in Saudi Arabia would have a TMI minimum to the north of the body and a maximum to the south, the peak is shifted northwards by the RTP process until it is directly over the body. This makes analysis of the data much simpler as RTP anomalies can be related more directly to the geology. The magnetic field inclination varied from about 40° in the north to less than 20° in the south, and the RTP processing allowed for this and also avoided any low-latitude problems. The first vertical derivative (FVD) of the RTP grid was calculated using Fourier methods and provides a derivative grid that enhances the higher spatial frequencies in the data, such as are caused by faults, intrusions, or contacts, and yields information about structure that may not be evident in the RTP grid.
Reduced-to-the-pole magnetic anomaly field
A conventional colour scale or spectrum with low magnetic values shown as blue and high values as red has been employed on the RTP maps generated by the Saudi Geological Survey, the appearance of which is enhanced by the application of artificial shading, where the direction of illumination is from the northeast. In the monochrome FVD plot, low (negative) values are dark, and high (positive) values are light. No scale is given for the FVD, because the monochrome map is best seen as an image enhancement process and the specific values are not usually of interest.
First-vertical derivative of the reduced-to-the-pole magnetic field
These regional-scale magnetic anomaly maps newly produced by SGS employ a single coordinate system to simplify use of the information. Most of the available magnetic data were originally plotted using coordinates based on the International Spheroid. To make the composite data set, all coordinates were converted to the WGS84 spheroid, which is used by the GPS and is commonly employed in modern surveying and exploration. The conversions used the constants for the Ain el Abd datum. A Lambert Conformal Conic (LCC) projection has been used for the grids and mapping, as this cartesian system is applicable over a wide range of longitudes and is appropriate for the scale of regional maps. The LCC projection employed has a central meridian of 48° E, which passes approximately through the center of the Arabian Peninsula, and therefore has been used for regional mapping. The two standard parallels are at 17° and 33° N. The latitude of projection origin is taken to be the equator, meaning that the origin (0, 0) is at 48° E and 0° N. The UTM projection, commonly used for larger scale maps than shown here, is not really appropriate for an area as large as that covered by the regional data sets reproduced because the projection may suffer appreciable distortion for areas outside each 6-degree wide zone.
Magnetics and geology
The magnetic data provide a large amount of information about the geology of central and western Saudi Arabia and adjacent parts of the Red Sea, showing structural elements such as faults, shear zones, sutures between Precambrian terranes, and Precambrian and Tertiary dikes, as well as prominent lithologic features such as plutons, dikes, and contrasted areas of magnetite-rich and magnetite-poor rock units.
Primary structures of the Arabian shield. In many cases these
correspond to magnetic anomalies. In other cases, the magnetic
anomalies indicate new structures not yet recognized in the field
The change in texture observed on some of the maps over areas of Cenozoic flood basalt (“Harrat”, in Arabic) is due to the larger flight line spacing utilized for surveys of the harrats (2,000 to 2,500 m) and a consequent loss of detail. Another feature to note is the abundance of anomalies with strong negative amplitudes (blue). Magnetic anomalies due only to induction in the present Earth's magnetic field are expected to be positive (red); the presence of intense negative anomalies is probably due to remanent magnetization in the reverse direction from the present field, a known characteristic of lenses of serpentinite along some shear zones in the shield.
As a general rule, the magnetic field is governed by the magnetic susceptibility of the crystalline rocks that make up the exposed shield, in western Saudi Arabia, and extensions of the shield beneath Phanerozoic rocks, in central Saudi Arabia. The Phanerozoic rocks of the Arabian Platform themselves, being sedimentary, are essentially magnetically transparent and, at the scale of the maps shown here, do not generate magnetic anomalies. Consequently, the magnetic field east of the exposed shield reflects the structure and lithology of the Precambrian rocks, the so-called magnetic basement.